First off, because this is all localized in your mind, none of us can really answer it for you. Only you can. I know that is not an answer you are looking for, but coming to grips and understanding one's own sexuality is something only YOU can do. That being said, there's absolutely nothing wrong about seeking advice, but realize that YOU are the ultimate arbiter of the matter no matter what anyone else says and no matter how forcefully they say it. You own your own mind and that includes your own sexuality and never cede control of it to anyone.
What part of gay sex turns you off? There are tons of gay and bi men who are not at all into anal sex. They enjoy oral sex and just plain exploring each other's bodies.
When you watch straight sex, what are you getting off on? Is it the guy only? Is it both? Do you fantasize about exploring her body inside and out (breasts, pussy, etc.)? Or is it more that you fantasize about being the other guy who is fucking the woman and the sexual pleasure you get is more about the guy than about gaining pleasure from fucking the woman?
When you look at pics of men and are turned on, do you have a desire to touch the guys?
I really don't have any great answers, and neither will anyone else on these boards, even if they seem to. But with these questions, perhaps you can begin to figure it out.
BTW some gay men do enjoy watching straight sex, but their focus is on the man.